The invention relates to a magneto-resistance device comprising two layers of ferromagnetic material mutually separated by at least one interposed layer of non-ferromagnetic material.
The invention also relates to a magnetic head employing such a device.
Magneto-resistance is that phenomenon whereby the electrical resistance measured along a given path within a suitable material is found to be influenced by the presence of a magnetic field intersecting that material. The phenomenon can thus be exploited to transcribe probed magnetic field variations into corresponding resistance variations within the material, thereby lending itself to application in magnetic field sensors and magnetic heads. Such magnetic heads are employed to transfer data to and from a magnetic recording medium, such as a magnetic tape or disc.
The magnitude of the magneto-resistance effect along a given path in a particular material is rendered by the formula: ##EQU1##
in which R.sub.L and R.sub.S are, respectively, the largest and smallest electrical resistances measurable along the said path in a variable magnetic field, at a given temperature. The value of MR is usually expressed as a percentage, and is preferably as large as possible so as to yield maximum attainable sensitivity in sensor applications such as those mentioned above.
The magneto-resistance effect in a layered structure can be investigated using at least two measuring geometries. The first such geometry is the so-called Current In Plane (CIP) geometry, whereby the electrical resistance of the device is measured using a method in which an electrical voltage gradient is applied in a direction substantially parallel to the plane of the device's constituent layers. The second such geometry is the so-called Current Perpendicular to Plane (CPP) geometry, whereby the electrical resistance of the device is measured using a method in which an electrical voltage gradient is applied in a direction substantially perpendicular to the device's constituent layers. For a given device, the magneto-resistance effect measured using the CPP geometry is generally larger than that measured using the CIP geometry, by as much as a factor of three. See, for example, the article by Gijs et al. in Phys. Rev. Lett. 70 (1993), pages 3343-3346.
A trilayer structure of the kind described in the opening paragraph is suitable for exploiting the so-called spin-valve magneto-resistance effect, whereby the resistivity of the structure is found to be influenced by the mutual orientation of the (net) magnetisations of the two layers of ferromagnetic material. The structure is then preferably embodied in such a way that an external magnetic field can be used to easily alter this mutual orientation. Such an embodiment can be achieved in at least three different ways, as follows:
(a) The two ferromagnetic layers can be antiferromagnetically coupled across the interposed non-ferromagnetic layer, so that their (net) magnetisations are mutually anti-parallel in the absence of an external magnetic field. A magnetic field applied parallel to the magnetisation of one of the ferromagnetic layers can then be used to reverse the magnetisation in the other ferromagnetic layer, thereby forcing both magnetisations into mutually parallel configuration; PA1 (b) One of the ferromagnetic layers can be exchange-biased to an additionally present anti-ferromagnetic biasing layer in such a manner that the magnetisation of the other ferromagnetic layer can be independently oriented by an external magnetic field, into either anti-parallel or parallel configuration with the magnetisation of the exchange biased layer; PA1 (c) The two ferromagnetic layers can be embodied to have considerably different magnetic coercivities. This can be achieved, for example, by the addition of differing concentrations of suitable foreign substances to the material of each ferromagnetic layer. A magnetic field of suitably chosen strength can then be used to separately orient the magnetisation of the ferromagnetic layer of lower magnetic coercivity, without changing the magnetisation-direction in the ferromagnetic layer of higher magnetic coercivity. PA1 (1) If the magnetisation vectors of the ferromagnetic layers are mutually parallel, then both these layers will behave as metals for electrons of a certain spin state (in the case of Fe.sub.3 O.sub.4, for example, this will be the spin-down state). Electrons of this particular spin state will thus be able to traverse the entire trilayer in a substantially perpendicular direction, whereas electrons of the opposite spin state will not. The electrical resistance in the former case will be essentially determined by the thickness and material of the non-ferromagnetic interposed layer; PA1 (2) If the magnetisation vectors of the ferromagnetic layers are mutually anti-parallel, then, for electrons of each spin state, one of these layers will behave as a metal, whereas the other will behave as an insulator. As a result, electrons of neither spin state will in this case be able to traverse the entire trilayer in a substantially perpendicular direction. The measured electrical resistance will thus be essentially infinite; PA1 (3) Statistically, a small current of electrons will always be able to tunnel through the entire trilayer structure, regardless of its magnetisation configuration. In addition, spin-flip scattering events can also occur in the interposed non-ferromagnetic layer, thereby causing the spin state of a small fraction of scattered electrons to undergo a reversal. In the case of an anti-parallel magnetisation configuration in the trilayer (point PA1 (2) above), an electron which has already traversed one of the ferromagnetic layers will, if it undergoes a spin-flip, also be able to traverse the second ferromagnetic layer, whereby a small leakage current arises. However, by ensuring that the interposed nonferromagnetic layer is relatively thin, such spin-flip occurrences are kept to a minimum. In addition, by making the ferromagnetic layers relatively thick, tunnelling events through the entire trilayer are limited. The magneto-resistance effect of the trilayer is then essentially 100%.
A magneto-resistance device of the type indicated in the opening paragraph is elucidated in an article by Nakatani and Kitada in J. Mat. Sci. Lett. 10 (1991), pages 827-828. In this known device, an electrically insulating Al.sub.2 O.sub.3 layer is interposed between two ferromagnetic Fe layers with in-plane magnetisation, and the magneto-resistance effect of the combination is measured using the CPP geometry. One of the Fe layers is doped with 1.7 at. % Ru in order to increase its magnetic coercivity, whereas the magnetic coercivity of the other Fe layer is reduced by doping its Fe with 2.0 at. % C. The resulting trilayer structure is therefore of the type i(c) elucidated above. The doped Fe layers are each 100 nm thick, whereas the interposed Al.sub.2 O.sub.3 layer is only 10 nm thick. Such a thin insulating layer acts as a tunnelling barrier between the conducting Fe layers which border it. In other words, even though the bulk material of the layer is an insulator, the thickness of the layer itself is small enough to allow a statistically significant number of electrons to tunnel across it.
The CPP electrical resistivity of this known trilayer device attains a minimum value when the net magnetisations of the two doped Fe layers are mutually parallel, and a maximum value when these two magnetisations are anti-parallel. Such spin-dependent tunnelling behaviour is rigorously discussed by Slonczewski in Phys. Rev. B 39 (1989), pages 6995-7002, where it is concluded (equation (6.1), page 7000) that the CPP conductance (G) of the trilayer satisfies the approximate relationship: EQU G=G.sub.0 (1+.epsilon. cos .theta.)
Here, G.sub.0 and .epsilon. are constants (.epsilon.&lt;1), and .theta. is the in-plane angle between the (net) magnetisation vectors of the two ferromagnetic layers. The fractional difference between the conductances of the trilayer for mutually parallel and mutually anti-parallel magnetisation configurations is therefore 2.epsilon..
A disadvantage of the spin-dependent tunnelling effect on which the known device is based is that the value of .epsilon. is generally very small. As a result, the room-temperature CPP magneto-resistance effect in that device is only about 1.0%. Such a low value greatly limits the sensitivity of potential magnetic field sensors employing the known device.
Another known spin-valve magneto-resistance device is disclosed in U.S. Pat. No. 5,134,533, in which layers of in-plane ferromagnetic material such as Fe, Co or Ni are antiferromagnetically coupled across interposed layers of electrically conductive non-ferromagnetic material such as Cr, V or Ti, thereby forming an alternating multilayer arrangement containing a plurality of the basic trilayer structures of the type (a) elucidated above. In this case, the electrical resistivity once again attains a minimum value when the net magnetisations of the two ferromagnetic layers are mutually parallel, and a maximum value when these two magnetisations are anti-parallel.
The magneto-resistance effect in this second known device is caused by spin-dependent scattering phenomena, whereby the extent to which conduction electrons are resistively scattered in the device is determined by the direction of their intrinsic spin in relation to the magnetisation-direction of the ferromagnetic layers. The effect is explained in more detail by Baibich et al. in Phys. Rev. Lett. 61 (1988), pages 2472-2475, in particular page 2474. Once again, the magnitude of the effect is quite small, so that the room-temperature CIP magneto-resistance of this second known device is only about 10%.